Velocity

Astigmatism Particle Image Velocimetry in microfluidics

Astigmatism particle tracking velocimetry is an optical method to determine the instantaneous volumetric position spherical particles in a volume of transparent medium using only a single camera view.  By introducing only a cylindrical lens into the optical system, the particle images appear as ellipses depending on their depth position within the measurement volume. Using the particle image’s width and height the particle position in 3D can be estimated. So every ordinary 2D imaging system can easily be enhanced to allow for 3D particle position measurements. The technique furthermore does not suffer from bias errors due to in-plane-spatial averaging and the out-of-plane averaging also known as depth of correlation effect. At the Bundeswehr University Astigmatism µPTV is nowadays our standard tool and helped to analyze many microfluidic applications as for example the electro thermally driven micro vortex seen in the figure on the right.

 

Funded by the Deutsche Forschungsgemeinschaft (DFG).

 

Partner:  Purdue University (Prof. Wereley), IFW Dresden (Dr. Uhlemann)

 

Contacts: Dr. Cierpka, Dr. Rossi

 

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Astigmatism-PTV Measurements of 3D Flow Fields in Compressors and Turbines

This project is part of the joint research program AG TURBO 2020 / 'Compression'. The goal of this program is to improve the efficiency of future turbo machines. This requires a precise knowledge of 3D engine flow fields. For the velocity measurements the APTV measurement technique is extended for application in turbo engines. APTV is capable of measuring volumetric flow fields with only a single camera, which is best-suited for measurement domains with a limited optical access. Furthermore, APTV has low vibration sensitivity and requires only low seeding concentrations. These are crucial features for measurements under challenging conditions. The flow fields can be determined in measurement domains up to a size of 70 x 30 x 30 mm³.
 

 

Partner:  MTU Aero Engines

 

Contacts: Dipl.-Ing. Fuchs

 

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Particle Image Velocimetry and Single-Pixel Ensemble-Correlation

The analysis of multi-scale flow phenomena over many orders of magnitude requires very large dynamics of the spatial resolution. For conventional window-correlation approximately 250 independent vectors can be determined for one direction from 4000 pixels, whereas single-pixel analysis offers a significantly increased dynamic spatial range of up to 2,000 independent vectors for one direction. This enables to resolve small scale structures and to obtain an overview of a larger field simultaneously. Although this method cannot reveal information about the current state of the turbulent processes, statistical quantities such as mean velocity field and Reynolds stresses can be estimated from the correlation function with high accuracy.

The significantly improved spatial resolution compared to window-correlation is of fundamental importance for the analysis of flows at high Mach and Reynolds numbers. Thus, single-pixel ensemble correlation is well suitable for the investigation of physical phenomena and for the validation of new numerical flow simulations. The figure shows the wake region of a generic space launcher model: the Reynolds shear stress distribution within the extremely thin shear layer determines the size of recirculation region and is therefore of particular interest for the validation of new numerical methods of the cooperation partners.

 

Funded by the Deutsche Forschungsgemeinschaft (DFG.

 

Partner: DLR Cologne (Dr. Gülhan), TU Braunschweig (Prof. Radespiel)

 

Contacts: Dr. rer. nat. Sven Scharnowski

 

 

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Particle Tracking Velocimetry

Particle tracking methods have a great potential for enhancing the spatial resolution and  measurement precision compared to correlation-based methods (PIV). They are not biased due to inhomogeneous seeding concentration or volume illumination and out-of-plane gradients. However,

at high seeding concentrations the reliable particle pairing is challenging and the measurement precision decreases rapidly due to noise caused by overlapping particle images. If the temporal sampling of the flow high enough, the particle image information acquired at four time steps can be used to improve the particle pairing at high seeding concentrations. Furthermore, it is shown that the accuracy can be increased by using vector reallocation and displacement estimation via a fit of the trajectory in the case of curved particle paths. 

 

Funded by the Deutsche Forschungsgemeinschaft (DFG) and the European Community (FP7/2007-2013) under grant agreement No. 265695.

 

Partner: TU Delft, DLR

 

Contacts: Dr. Christian Cierpka, Dr. Alvaro Marin

 

 

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Volumetric reconstruction of fluid interfaces using 3D PTV

Tracer particles are commonly used in experimental fluid dynamics to probe the velocity of a flow. It has been recently demonstrated how this approach could be extended to successfully reconstruct the interface between two fluid phases or a fluid/gas interface. This approach has some advantages compared with other methods traditionally used (such as the dye visualization) since the diffusion coefficient of tracer particles is considerably smaller than to the one of molecular dyes and the 3D topology of an interface can be obtained with a single recording (using 3D-PTV), with no need of scanning procedures.  The main limitation of this approach is due to the resolution of the reconstruction that is constrained to the tracer particle concentration. The method has been already successfully applied to characterize the topology of liquid-liquid interfaces, using the Astigmatism Particle Tracking Velocimetry as 3D PTV method. An ongoing research is currently carried out in this domain, in order to improve the accuracy and resolution of the method and to extend it to a wider range of applications (free-surfaces, droplets, jets, bubbles). 

 

Funded by the Deutsche Forschungsgemeinschaft (DFG).

 

Partner: Politecnico di Torino

 

Contacts: Dr. M. Rossi

 

 

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